Semiconductor optical amplifiers (SOAs) are forward-biased diodes that provide optical amplification or gain for an input optical signal through stimulated emission, a process fundamental to the operation of lasers. Amplification is achieved by injecting electrical current or carriers (electrons and holes) into the SOA's active region. Carrier injection creates a population inversion between the conduction and valence bands which causes the input signal's intensity to increase as it passes through the SOA. Amplification occurs for a range of wavelengths near the band gap of the semiconductor.
Semiconductor optical amplifiers can also be used as all-optical wavelength converters for broadband wavelength-division-multiplexing networks as described by Yoo in J. of Lightwave Technology, vol. 14, p. 995 (1996). A wavelength converter device is an optical modulator that transfers the intensity modulation from one optical wavelength to a CW optical signal at a different wavelength. The optical digital signals commonly used are either non-return-to-zero (NRZ) where the intensity does not go to zero between continuous 1 bits and return-to-zero (RZ) where intensity does go to zero between continuous is as shown in FIGS. 1 and 2.
All-optical wavelength conversion is achieved either by cross-gain modulation in a single amplifier or by cross-phase modulation in one or two amplifiers integrated in an interferometer as explained in T. Durhuus et al., J. Lightwave Technology, vol. 14, p. 942 (1996). In the cross-gain-modulation technique, shown schematically in FIG. 3 for NRZ data, optical data at wavelength .lambda..sub.1 and a CW optical signal at wavelength .lambda..sub.2 enter a semiconductor optical amplifier 21. A band-pass optical filter 22 passes only .lambda..sub.2 at the output. The output is transmitted through a length of optical fiber 23 and detected by a receiver 24. The data signal modulates the gain of the amplifier as it is reducing the population inversion by stimulated emission during the 1 bits. This modulates the CW signal, producing a logically inverted copy of the data; A 1 bit for .lambda..sub.1 means lower gain for .lambda..sub.2, hence a 0 at .lambda..sub.2 and vice-versa. The inversion is one disadvantage of this method. Also when applied to RZ data, this scheme requires a train of pulses instead of the CW input, which requires an additional clock pulse source 25 as shown in FIG. 4.
In one of the cross-phase modulation schemes, shown in FIG. 5 two SOAs 26 and 26' are integrated into the arms of a Mach-Zehnder interferometer 27, forming an optical modulator. The band-pass filter 22 passes only .lambda..sub.2 at the output. The interferometer is adjusted as to have a null output with no data input. The data pulses modulate the refractive index of the SOA in one arm by modulating its gain. The interferometer is now unbalanced and gives an output at .lambda..sub.2 duplicating the intensity modulation of the data at .lambda..sub.1. As compared with cross-gain modulation, this device, and other interferometer versions of it, have the advantage of a higher extinction ratio (ratio of 1s to 0s) and non-inverting outputs. Also, when applied to RZ data, clock pulses are not required; a CW signal at .lambda..sub.2 is sufficient.
The finite gain recovery time of the SOA ultimately limits the modulation speed of both schemes for wavelength conversion. A characteristic roll-off frequency for optical modulation transfer function of the devices can be determined as described by T. Durhuus et al., J. Lightwave Technology, vol. 14, p. 942 (1996). At bit rates higher than the roll-off frequency, the converted output signal at .lambda..sub.2 is distorted; the intensity of the 1 bits varying depending on the bit pattern that preceded them. The roll-off frequency can be increased somewhat by amplifier design, increased amplifier length, and higher optical powers. Also cascading two amplifiers as described by S. L, Danielsen, et al., Electron. Lett., vol. 32, pp. 1688-1690, (1996) has achieved operation at 40 Gb/s.
However, this comes at great cost and requires specially designed amplifiers. For photonic integration of SOA wavelength converters with other passive and active components, the high modulation speed of the SOA has to be compromised to facilitate the fabrication process. This reduces the frequency roll-off of the wavelength converter, as for example in the device reported by L. H. Spiekman et al. IEEE Photons. Technol. Lett., vol. 9, pp. 1349-1351 (1997). Also with present devices there is no simple way for upgrading the device's operation speed significantly without replacing it with a new device with higher frequency roll-off.
Another technique applicable to return-to-zero (RZ) data for wavelength conversion in a semiconductor optical amplifier is described by D. M. Patrick and R. J. Manning in Electron. Lett. vol. 30, pp. 252-253 (1993). Optical data of RZ, format at .lambda..sub.1 and a CW signal at .lambda..sub.2 co-propagate in the SOA. A birefringent filter is used to separate the spectrally shifted components of the CW signal and produce pulses at .lambda..sub.2. This discriminator has two major drawbacks. It is sensitive to the polarization of signal at .lambda..sub.2, requiring adjustments by polarization controllers which would also have to be stabilized. The transmission of a birefringent filter is limited to being sinusoidal, which is the not ideal filter function for digital applications Also because of the sinusoidal response, the steepness of the filter is tied to its bandwidth. Furthermore the birefringent filter includes 100 m of birefringent fiber, two polarization controllers and two free-space optical polarizers, which makes the filter bulky and lossy.